8-10 October 2018
INFN - Laboratori Nazionali di Legnaro
Europe/Rome timezone

Two different origins of Q-slope problem in a Nb film cavity for HIE-ISOLDE

8 Oct 2018, 16:40
Villi Room (INFN - Laboratori Nazionali di Legnaro)

Villi Room

INFN - Laboratori Nazionali di Legnaro

Viale dell'Università 2, Legnaro (Padova) - Italy
Theoretical modelling of RF behavior Theoretical modelling of RF behaviour


Dr Akira Miyazaki (CERN and the University of Manchester)


Since their invention, the performance of Nb film cavities has always been limited by a strong Q-slope. Recent tests at CERN of the newly developed seamless Quarter-Wave Resonator (QWR) showed an interesting decomposition of the Q-slope, which might help to explain/cure this limitation. One is caused by non-linear trapped flux oscillation in the residual component, whose surface resistance depends linearly on the RF field strength. The other is the medium-field Q-slope, which depends exponentially on temperature and RF field. Once the trapped flux is suppressed by using magnetic field compensation during cool down upon superconducting transition, the cavity's Q-slope at superfluid temperature was as small as for bulk Nb cavities up to peak magnetic fields of 120 mT. We argue that the Q-slope problem in low-frequency Nb film cavities originates from environmental conditions. These cavities are typically operated in normal fluid without magnetic shielding. Thermal gradients in the previous design of HIE-ISOLDE QWRs may give rise to thermoelectric currents during cool down, and thus dominate trapped-flux induced Q-slope in the residual resistance. We also study the fundamental cause of medium-field Q-slope and compare it with several models. Reducing the Q-slope of Nb coated cavities would make them an interesting and cost-effective alternative to bulk Nb cavities for an increased number of applications.

Primary author

Dr Akira Miyazaki (CERN and the University of Manchester)

Presentation Materials